1. Field of the Invention
The present invention relates to a method of improving the fracture-mechanical properties of highly scratch-resistant, radiation-curable coating systems, to coatings obtainable by said method, and the use of such coatings.
2. Description of the Background
High scratch resistance is a widespread requirement of coatings. It is met, for example, in the automobile industry by covering plastics parts with a scratch-resistant clearcoat, which may be composed on the one hand of thermosetting PU coating materials referred to as one- or two-component PU coating materials or preferably, on the other hand, of clearcoat materials which cure on exposure to actinic radiation. In the case of hard coatings, however, a problem which frequently arises is that a microcrack initiated in the coating propagates with a very high degree of local definition through the coating into the substrate to which the coating material is applied, as is described, for example, in DE-A1 199 56 483.
DE-A1 199 56 483 describes coated polymer moldings which, on a polymer layer comprising at least one graft polymer formed from a rubber-elastic graft base having a glass transition temperature, Tg, of below 10° C. and a graft having a Tg of more than 30° C., have at least one coating-possessing certain properties. A rubber phase is therefore present distributed in the substrate.
A disadvantage of the process described is that the extent to which such graft polymers can attach covalently to the topcoat is inadequate, since grafting can take place only by way of the volume fracture of the elastomeric component of the blend. For a coating system with high fracture resistance in particular it is important to prevent small microcracks spreading through a continuous elastomer coat.
DE-A1 199 20 801 describes special multicoat clearcoat systems in which each clearcoat film is radiation-curable, one coat being made scratch-resistant by means of inorganic nanoparticles.
DE-A1 100 27 268 describes special multicoat clearcoat systems in which curing takes place in one step by means of thermal crosslinking of polyols based on acrylic copolymers with triazine crosslinker; this may occur simultaneously or successively.
WO 99/26732 describes a process for preparing a multicoat paint system in which a coating material and a clearcoat film are applied to a substrate, which may have been coated beforehand.
A disadvantage of these cited texts is that even when the technical teaching disclosed therein is employed the scratch resistance, in particular the problem of cracking within the coating, is unable to satisfy the present-day requirements.
It is an object of the invention to develop a coating which has very high scratch resistance with good adhesion to the substrate at the same time as reduced crack propagation into the substrate.